The invention relates generally to the field of well logging instrumentation using high voltage operated energy sources. More specifically, the invention relates to electrical insulators used with such well logging instrumentation when the insulation is exposed to ionizing radiation.
Certain well-logging instruments, for example, pulsed neutron devices and x-ray emitting devices, require the use of very high voltages within relatively small and confined spaces, at high temperatures and in the presence of ionizing radiation. In such well logging instruments, the components operated at high voltage are located near ground potential components, such as the instrument housing. The high voltage operated components and the ground potential components are electrically isolated from each other using insulation that can occupy a tightly confined space. Evaluation of such insulation, even insulation having higher than the required dielectric strength when initially placed into service may fail over time (often catastrophically in just a few hundred hours' operating time). This has been shown especially to be the case if the well logging instrument, while operating at a high ambient temperature, produces ionizing radiation or may operate in the presence of externally produced ionizing radiation.
Experiments performed repeatably several times have shown that under certain conditions, insulating sleeves known in the art used with pulsed neutron generators (“PNGs”) can fail catastrophically within a few hundred (˜400-600) hours of PNG operating time. The tested insulating sleeves were double layer sleeves with the required initial dielectric strength and sleeve thickness. The first visible indicia of insulating sleeve failure were sudden current spikes (arcs) inside a chamber that houses the PNG and its high voltage (“HV”) power supply, such arcs occurring many hours apart. Once the arcs became more frequent, PNG operation was stopped and the chamber was opened. At certain points adjacent to the HV end of the inner insulating sleeve, the tested insulating sleeves had degraded enough to show a multiplicity of burned tracks. One example of such degraded sleeves is shown in FIG. 1A. FIG. 1A is a photograph of the outer layer of the insulating sleeve, the outer surface of which faced the wall of the grounded instrument housing. The burn tracks are highly concentrated at the outer surface, forming “tree trunk” like structures that then begin to branch out slowly toward the inner surface of the insulating sleeve. FIG. 1B is a photograph of the inner layer of the insulating sleeve, the inner surface of which faced the HV end of the PNG. The burn tracks are somewhat concentrated at the outer surface, forming “thick branch” like structures there and then branch out towards the inner surface into a widely diverging maze of small branches.
It is useful for HV well logging instrument designers to understand how and why insulating sleeve damage occurs. It is desirable to increase the useful lifetime of an insulating sleeve by means other than making the insulating sleeve thicker and/or using a higher intrinsic dielectric strength material, since both of the foregoing parameters already are near their practical maxima to meet the operating requirements of well logging instruments known in the art.